1. Field of the Invention
The present invention relates to an optometric chart presenting apparatus used in visual acuity testing and the like.
2. Description of Related Art
One conventionally known optometric chart presenting apparatus is provided with binocular sight windows which eyes to be examined look into. Optotypes are presented to the eyes through these sight windows within a casing and the eyes are made to observe the optotypes so that visual function of the eyes such as visual acuity is examined. Conventionally, this type of apparatus presents optotypes in the following three ways.
(a) One of various optotypes illustrated on a rotating optometric chart disk or the like is placed in the center of right and left optical paths, and a luminous flux of an optotype is guided to right and left sight windows with the use of a prism or a mirror. As the result, the optotype can be presented to the both eyes. The testing distance is adjusted to a predetermined distance for far vision by a convex lens disposed at some midpoint along the optical paths.
(b) Similarly to the above mentioned way (a), one of the optotypes is placed in the center of the right and left optical paths. In addition, large convex lenses are disposed to cover each of the sight windows for right and left. Due to this configuration, it is possible to present the optotype to the both eyes optically at a predetermined distance for far vision.
(c) Right and left optical paths are configured to be independent of each other and an optotype presentation window is disposed to each path. By placing same optotypes alternatively in each of the windows, the optotypes are presented to the both eyes. In this way, images of the optotypes look fused into one image on binocular observation. The testing distance is adjusted to a predetermined distance for far vision by respective convex lenses disposed at some midpoint along each of the optical paths for right eye and for left eye (at each sight window, for example).
However, the ways of presenting optotypes as described above have following disadvantages. That is, the above-mentioned way (a) requires optical elements such as a prism and a mirror for guiding one luminous flux of an optotype to the both eyes, which inevitably results in higher cost of the apparatus. Also, the above-mentioned way (b) requires a large-sized convex lens, which also raise the cost.
According to the above-mentioned way (c), although the apparatus can be comprised of minimum optical elements, it needs to be configured to present the same optotypes in both right and left presenting windows. To meet this end, the apparatus may be configured to individually place same optotypes alternately in each presenting window with the use of two rotating optometric disks. Yet, in the case of rotating the two rotating optometric disks individually, it is extremely difficult to stop the rotating optometric disks in such a manner that optotypes for right and for left come in the same relative positions to the respective windows. If the optotypes for right and for left are stopped at different relative positions, images of the optotypes may not be fused into one at the time of observing the optotypes with both eyes. As the result, examinations of visual function can not be carried out accurately. In addition, provision of two rotating disks and motors to make the rotation results in higher cost and a larger apparatus.
Further, presenting far vision optotypes (far vision examination) and presenting near vision optotypes (near vision examination) by this type of apparatus are carried out owing to the following configurations.
For far vision measurement in the case of above way (c), the convex lenses disposed at the right and left sight windows functions to present optotypes optically at a predetermined distance for far vision in the casing. For near vision measurement, by removing the convex lenses that are disposed at the sight windows for right and left from the optical paths, or by replacing the lenses, optotypes are presented- at a predetermined distance for near vision in the casing.
However, the above type of apparatus has disadvantages as follows. That is, it is configured that an examinee observes optotypes presented ahead of him through the same sight windows in both types of measurement for far vision and for near vision. Due to this configuration, it is nearly impossible to measure near vision of a patient wearing a progressive addition lens, a bifocal lens or the like on downgaze. It is true that the patient manages to look at optotypes in the front through a near portion of the progressive addition lens or the bifocal lens by turning his face upward to tilt his head. Yet, the patient is forced to take an uncomfortable position and therefore accurate measurement can not be achieved.
In addition, when going into near vision measurement after far vision measurement, the lenses disposed at the sight windows need to be switched over. The need for a switching-over mechanism required therefore increases complexity of the configuration and also increases trouble to carry out the operation if it is done manually.